1. Field of the Invention
The present invention relates to a novel isocyanate composition, microcapsules made from the isocyanate composition and a recording material including the microcapsules, and a production method of the microcapsules.
2. Description of the Related Art
Polyvalent isocyanate compounds are widely used as raw materials for polyurethanes, polyurea resins and urethane elastomers. Polyvalent isocyanate compounds are also useful as a wall material for producing microcapsules by interfacial polymerization. The resultant microcapsules are used in pressure sensitive recording materials, thermosensitive recording materials and adhesives.
Materials obtained by applying a solid dispersion including mainly an electron-donating dye precursor to a substrate and drying the resultant coating are used as thermosensitive recording materials, which prevail as recording media for facsimiles and printers. Recording methods which include the use of electron-donating dye precursors are advantageous in that materials are readily available and exhibit high color development concentration and high sensitivity to heat. However, undesired coloring easily occurs depending on the storage conditions after recording, application of heat or adhesion of solvent. In other words, the recording materials have problems regarding storability and reliability of recorded images. Accordingly, research to solve such problems has been ernestly conducted.
As a method for improving storability of recorded images, a method was suggested in which storability of images is enhanced by encapsulating the electron-donating dye precursor and isolating the dye precursor from a developer in a recording layer. This method can provide a high color development ability and image stability.
Besides the above-mentioned thermosensitive recording materials, thermosensitive recording materials utilizing diazonium salt compounds, so-called diazo-type thermosensitive recording materials, has been put to practical use. Diazonium salt compounds react with a coupler such as a phenol derivative, or a compound having an active methylene group to form a dye. However diazonium salt compounds also are photosensitive, and so, when irradiated with light, they lose the above reactivity. Because of such characteristics diazonium salt compounds have been recently applied to thermosensitive recording materials, and photo-fixed thermosensitive recording materials are proposed, in which, when the materials are heated, a diazonium salt is caused to react with a coupler so as to form an image, and the image is then fixed by irradiating the materials with light (e.g., see Koji Sato et al., “Journal of the Institute of Image Electronics Engineers of Japan” (Vol. 11, 4th, pp. 290-296, 1982).
However, recording materials including a diazonium salt compound have high chemical activity and diazonium salt compounds gradually react with couplers even at low temperature, giving the disadvantage of a short shelf life of the recording materials. In order to solve this problem, a method was suggested, in which a diazonium salt compound is encapsulated in microcapsules to separate the compound from couplers, water and basic compounds (e.g., see Tomomasa Usami et al., “Journal of the Image Society of Japan” (Vol. 26, 2nd, pp. 115-125, 1987).
Multicolor thermosensitive recording materials have also drawn attention as applicable to the field of thermosensitive recording materials. Reproduction of a multicolor image by using thermosensitive recording was thought to be more difficult than by using electrophotographic recording or ink-jet recording. However, it has become apparent that multicolor thermosensitive recording materials can be obtained by laminating, on a substrate, two or more thermosensitive recording layers including, as main components, an electron-donating dye precursor and a developer. Alternatively, two or more thermosensitive recording layers including a diazonium salt compound and a coupler which, when heated, reacts with the diazonium salt compound to develop color can be used. In order to obtain excellent color reproductibity, such multicolor thermosensitive recording materials require precise control of the heat responsiveness of the microcapsules by which, when heated, the microcapsules allow materials inside and/or outside the microcapsules to permeate the microcapsules.
An example, which is well known, of a method for encapsulating in microcapsules an electron-donating dye precursor or a diazonium salt compound is the following. An electron-donating dye precursor or diazonium salt can be dissolved in an organic solvent (the oil phase), and then added to an aqueous solution (aqueous phase) including a water-soluble polymer, and then emulsification-dispersed. By adding a wall material monomer or prepolymer, to either the resultant oil phase or aqueous phase, polymer walls serving as microcapsules can be formed at the boundary between the oil phase and the aqueous phase (e.g., see Tomoji Kondo, “Microcapsules” (The Nikkan Kogyo Shimbun, Ltd., 1970) and Yasushi Kondo et al., “Microcapsules” (Sankyo Shuppan, 1977)). The microcapsule walls can be made of gelatin, alginate, celluloses, polyurea, polyurethane, melamine resin or nylon. Polyurea and polyurethane resins are particularly preferably useful in the design of thermosensitive recording materials, because they have a glass transition temperature in the range from room temperature to about 200° C., which enables the capsule walls to show heat responsiveness.
In order to obtain microcapsules having polyurethane or polyurea walls, a production method conventionally conducted is the following. A diazonium salt or an electron-donating dye precursor is dissolved in an organic solvent, a polyvalent isocyanate compound is added to the resultant solution, and the resultant organic phase solution is emulsified and/or dispersed in an aqueous solution including a water-soluble polymer. A catalyst to accelerate polymerization is added to the aqueous phase or the temperature of the resultant emulsion is raised to cause the polyvalent isocyanate compound to react with a compound having an active hydrogen, such as water, and to form capsule walls.
As the polyvalent isocyanate compound, which is one of the raw materials of polyurea or polyurethane walls, for example, an adduct of 2,4-tolylenediisocyanate and trimethylolpropane, or an adduct of xylylenediisocyanate and trimethylolpropane are known (e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 62-212190 and 04-026189).
However, even having polyurea or polyurethane capsule walls, one of raw materials of which is the polyvalent isocyanate compound, insufficiently lengthens the shelf life of recording materials including diazonium salt compounds. When thermosensitive recording materials having insufficiently long shelf life are stored, for example, at a high temperature and a high humidity before actual use thereof, they undesirably develop coloring called “fogging” of the background thereof, which deteriorates the visibility of recorded images. One answer to such a problem is to thicken the microcapsule walls. However, this solution deteriorates the color development sensitivity during thermal recording. Accordingly, it is extremely difficult to attain both high color development ability whilst further increasing shelf life.
As one method for solving such a problem, a method using a product obtained by causing a part of the polyvalent isocyanate compound to react in advance with a monoalcohol compound is known (e.g., see JP-A No. 05-317694). However, specific examples of the only monoalcohols having about 2 to about 9 carbon atoms are used in this method. In addition, when the proportion of alcohol is increased, sensitivity does increase but “fogging” also increases. “Fogging” can be prevented by decreasing the proportion of alcohol, but sensitivity becomes insufficient.
In addition, multicolor thermosensitive recording materials have cyan, magenta and yellow colors-forming thermosensitive recording layers, and respective color images are recorded on the materials by heating the materials to different corresponding temperatures. Therefore, the recording layers of multicolor thermosensitive recording materials are required to have more precise heat responsiveness than those of conventional thermosensitive recording materials. However, the above-mentioned conventional polyurea and polyurethane capsule walls do not satisfy this requirement.
It is known that addition of a heat sensitizer to the color development layer of a thermosensitive recording material improves sensitivity to heat. It is known that p-toluene sulfonamide, a heat sensitizer, show good performance (e.g., see Japanese Patent Publication (JP-B) No. 06-055546). In addition, it is known that arylsulfonamide compounds having specific substituent(s) shows better performance (e.g., see JP-A No. 09-039389). When the above arylsulfonamide compounds are used in multicolor thermosensitive recording materials, it is necessary to emulsify the arylsulfonamide compound to decrease haze of the thermosensitive color development layer. A method for emulsifying the arylsulfonamide compound is not specifically limited, and can be a conventionally known method. Specifically, an emulsion is made by dissolving the arylsulfonamide compound in an organic solvent hardly soluble or insoluble in water, mixing the resultant solution with an aqueous phase including a surfactant and/or, as a protective colloid, a water-soluble polymer and stirring the resultant mixture (e.g., see JP-A No. 02-141279).
The heat sensitizer is generally a crystalline substance. Therefore, when the emulsion including the heat sensitizer is stored for a long period of time, problems such as the crystals precipitating out from the emulsion may occur. Accordingly, microcapsules having sufficient sensitivity to heat without the use of such a heat sensitizer or only the use of a small amount of such a heat sensitizer are strongly desired.
From these viewpoints, heat responsible microcapsules made from a polymer obtained by polymerizing an isocyanate compound including an adduct has been suggested (e.g., see JP-A No. 10-114153). In this case the adduct is a compound (A) which has one active hydrogen in the molecule thereof and which has at least one chain with an average molecular weight in the range of 500 to 20000 from the group of a polyether chain, a polyester chain and a (co)polymer chain of a vinyl monomer. Alternatively, the isocyanate compound is a multifunctional isocyanate (B) having two or more isocyanate groups in the molecule. The publication listed above describes that microcapsules capable of suppressing background fogging and yet having high heat responsiveness can be provided. In addition, microcapsules having polyurea shells whose raw materials include an isocyanate that is a product of at least multifunctional isocyanate and a monovalent polyoxyethylene alcohol are disclosed (e.g., see Japanese Patent No. 3,266,330), and this publication describes that small microcapsules can be provided at low cost for emulsification.
However, although JP-A No. 10-114153 discloses a vinyl monomer as an example of the compound (A), it does not mention specific particular effects which derive from the type of vinyl monomer used. In addition, the publication does not mention at all a vinyl monomer including polyether, which is one of the constitutional elements of the invention.
Accordingly, there is a need for an isocyanate composition that can be used to produce supperior microcapsules suitable for use in thermosensitive recording materials and provides both excellent pre-recording storability (long shelf life), and also is able to produce excellent color development ability by giving good contact of coupler and developer in response to heat.
Also there is a need for a thermosensitive recording material including such microcapsules, having high sensitivity, high developing properties and excellent pre-recording storability (long shelf life). There is a need for a multicolor thermosensitive recording material having high sensitivity, excellent color reproducibility and excellent pre-recording storability. There is a need for a method for forming the above-mentioned useful microcapsules.